Currently, nitric acid is produced industrially via the catalytic oxidation of ammonia, over a platinum or platinum alloy-based gauze catalyst. This process, known as the Ostwald process, has essentially remained unchanged, since its inception in the first decades of the twentieth century. Ostwalds's patent was dated 1902 and when combined with Haber's development of synthesising ammonia, in 1908, the basis for the commercial production of nitric acid, which is used today, was in place.
The combustion of ammonia is carried out over a platinum based metal or alloy catalyst in the form of a gauze or mesh or net. A number of gauzes are installed together, and they constitute the gauze pack. The upper-most gauzes have compositions optimised for the combustion of ammonia, and are referred to as the combustion gauzes. Gauzes with other compositions may be located below the combustion gauzes, and these may have other roles, as described below. The whole stack of gauzes is referred to as the gauze pack. The gauzes are produced either by weaving or knitting.
The operating temperatures of the plants are typically 830 to 930° C. and the range of pressures is from 100 kPa to 1500 kPa. Typically, the combustion gauzes are installed in the plant for between six months and two years, depending on the plant operating conditions. Plants operating at high pressures typically have shorter campaigns than low-pressure plants.
The duration of the campaign is governed by a loss in the selectivity of the catalyst, towards the desired nitric oxide product, through the increased formation of unwanted nitrogen and nitrous oxide by-products. The loss of selectivity is related to a number of phenomena. During combustion, platinum is lost through the formation of PtO2 vapour. Some of the platinum may be recovered by the installation of palladium metal based gauzes, directly below the platinum based combustion gauzes. The PtO2 vapour alloys with the palladium, therefore, platinum is retained in the catalytically active zone. However, due to the depletion of platinum in the upper combustion zone of the gauze pack, not all of the ammonia is immediately combusted. If the ammonia is combusted in the palladium gauze region, the selectivity towards nitric oxide is reduced, and secondly, if ammonia and nitric oxide coexist in the vapour phase for a period of time, nitric oxide is reduced by ammonia, through a homogeneous reaction. This leads to both nitric oxide and ammonia losses. A final mechanism for loss of selectivity is related to the fact that the platinum is lost from the combustion gauzes at a higher rate than the other alloying elements (typically rhodium). This leads to rhodium enrichment of the gauze surface which leads to selectivity loss.
Over the last sixty years, many attempts have been made to replace the expensive platinum-based combustion catalyst with a lower cost catalysts, based for example on metal oxides. To date, the only commercially available oxide based catalyst for ammonia combustion, was developed by Incitec Ltd (Australia). This is based on a cobalt oxide phase. However, in terms of its selectivity of combustion of ammonia to the desired nitric oxide product, its performance is inferior to that of platinum-based systems. The cobalt oxide based systems have shown selectivity levels of circa 90%, in commercial units, compared to the 94 to 98% achieved with platinum based catalysts.
The use of mixed oxides with the perovskite structure, such as rhombohedral lanthanum cobaltate, as catalysts for ammonia oxidation, has received much attention. However, when considering the conditions that the catalyst is subjected to in industrial ammonia oxidation, it can clearly be seen that they are not suitable for stability reasons. Ammonia oxidation on an industrial scale, takes place at temperatures from 830 to 930° C. and at pressures from 100 kPa to 1500 kPa. The concentration of ammonia is in the range of 8.5 to 12 mol %, depending on plant conditions, with the remainder of the gas consisting of air. Thus the gas feed for oxidation has a composition of approximately 10 mol % NH3, 18.7 mol % O2 and the balance being nitrogen. When the ammonia is oxidised to NOx (NO+NO2), with an efficiency of 95%, the gas composition is approximated by 9.5% NOx, 6% O2 and 15% water vapour. (The balance of gas composition is nitrogen and some 800 to 2000 ppm of N2O). Thus the ammonia oxidation catalyst is subjected to high temperatures and a gas environment that contains oxygen and water vapour. These are the ideal conditions for the evaporation of metal ions, in the form of hydroxides and oxyhydroxides. Thus material will be lost from the catalytic reaction zone as vapour phase species, which will in turn be deposited downstream in a cooler zone of the reactor system.
If considering evaporation from mixed oxides (those that contain more than one metal component), it most often has an incongruent evaporation process. This is the situation where one component in the oxide has a higher evaporation rate than another or than the others. If considering the lanthanum cobaltate perovskite system, when heated in an atmosphere containing oxygen and water vapour, cobalt species, such as CoOOH, have much higher vapour pressures than the dominant lanthanum species La(OH)3. The effect of this is that cobalt evaporates to a larger extent than lanthanum, thus incongruent evaporation. The result of preferential cobalt evaporation is that in time, the non-stoichiometry limit of the lanthanum cobalt perovskite X will be exceeded (LaCo1-XO3 where X and 0<X≈<0.03). When the limit is exceeded, La2O3 will be precipitated. When operating, La2O3 does not have a negative effect on the catalyst performance. However, when the plant is shut-down or when it trips, the oxide catalyst is exposed to the ambient air. On cooling in air, the free La2O3 will hydrate; forming La(OH)3. 1 mole of La2O3 will form 2 moles of La(OH)3, which involves a 50% expansion of the volume of the free lanthanum species. This results in a mechanical disintegration of the catalyst.
Different perovskite type oxidation catalysts are known for use in different oxidation reactions. Examples of such catalysts and reactions are mentioned below.
Pecchi, G et al., “Catalytic performance in methane combustion of rare-earth perovskites RECoo,50Mn0,50O3 (RE: La, Er, Y)”, Catalysis today 172 (2011) page 111-117. This article describes physic-chemical properties for compounds where Co and Mn are present in equimolar quantities. The catalytic activity is related to methane combustion.
Russian patent RU2185237 describes catalysts for use in ammonia oxidation. The active catalyst is a composition with perovskite structure of the formula Mn1-xR1+xO3, wherein R=Y, La, Ce or Sm and X=0−0.596. A catalyst support of alumina is used. However, this patent describes a method of producing N2O, which is used in various areas as in semiconductors, perfume industries, in medicine and food industry. The catalysts show increased activity and selectivity for N2O and low selectivity for NO, which is the opposite of what is wanted for nitric acid production.
EP 532 024 relates to a catalyst for catalytic reduction of nitrogen oxide. More particularly, it relates to a catalyst for reduction of nitrogen oxide using a hydrocarbon and/or an oxygen-containing organic compound as a reducing agent, which is suitable for reducing and removing harmful nitrogen oxide present in emissions from factories, automobiles, etc. It is used a perovskite type compound oxide on a solid carrier. This catalyst selectively catalyses a reaction of nitrogen oxide with the reducing agent so that nitrogen oxide in emissions can be reduced efficiently without requiring a large quantity of the reducing agent.